A power station (601-03-01, also referred to as a generating station, power plant, or powerhouse) is an industrial facility for the generation of energy (electricity, useful heat...).
This page explains how we can map all power plants around the world, distinguished in two classes : enclosed and dispersed.

Please beware to not use power=transformer in this context : these devices are not designed to produce electricity but only to step up or down the voltage.

The rated output of the whole facility often available in operator public documentation. The tagging is consistent with generator:output=*: a co-generation plant would have for example plant:output:electricity=* and plant:output:steam=* keys. They accept numeric values or yes if the exact rating is unknown.

The aggregate of fuels used by individual generators within the plant. Multiple fuels should be separated by semicolons (;). Example: plant:source=coal;gas

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In this case, spatial capabilities of the database would enable us to retrieve features which are inside the power plant's perimeter.
According to the functional chart, we must find object tagged as power=generator + generator:plant=output inside and attributes must be defined consistently with power plant's ones.

Many other stuff (buildings, substations), introduced all along this proposal is recommended or optional.

We can compare the total rating of a power plant, specified in plant:output=* and addition of all the generator:output=* values of its generator members :

Please note that this comparison is made on two axis : The first by comparing all kind of output in both plant and generators (it's a problem to have a plant with yes whereas there is no generator with generator:output:electricity=* for instance).
The second axis is on the amount of each kind of power made as above rules.

A simple rule to make the distinguishing between operators is to create a separate area for each one. Two or more power plants would directly be created.

In general, only one company is operating a power plant so if two or more operators are present on a site, we could have two different power plants or two different components for power generation.
In all cases we mustn't map it in a single one area. Each area will have different operator=* values and potentially as many name=*.
The key point is still the ownership of the land occupied by the power plant.

Furthermore and often, room is reserved besides actual power plants for future extensions. This room should be concerned by the power plant area right now since it's fenced as the main production site.

Finally, Relation:multipolygon doesn't match these operator consideration since operator distinguishing MUST be done.

Dispersed facilities power plants

Functional chart of a dispersed power plant

Lillgrund Wind Farm, Sweden

Wind, tidal and photovoltaic power plants may consist in a group of several turbines or solar panels in the same location used for production of electricity.
Each turbine or panel would be tagged as power=generator.
The land between generators may be used for agricultural or other purposes while it may correspond to the ocean's ground for tidal farms.
A wind/solar farm may also be located offshore instead of a tidal farm which is always located on the sea ground.
When there are several fenced areas around the power plants sites, Relation:multipolygon should be used.

A 34ha solar farm producing 24 MW of electricity when sun comes up in south of France. With a little aerial imagery help, generators are taken as groups of solar cells for sake of simplicity. It is actually possible to use for each photovoltaic module but it's really expensive in mapping time.

The relation accepts two closed and fenced perimeters, all generators and a transmission substation connecting to the power grid

Hydraulic

Boulder dam is a famous dam built in early 1900's next to Las Vegas, USA. The power plant is located just under the dam and conected to the Lake mead with huge penstocks digged in the surrounding bedrock. A substation collects the 19 generators power as to transmit to the power grid. The lake also allows authorities to feed nearby towns with fresh water.

Since each component isn't linked by a fenced perimeter, a relation is used to make this power plant alive in OSM.

Genissiat dam is a run-of-the-river power plant relying on a big concrete structure to get a 23km long lake on the Rhone river. It is hosting 6 power generators inside the dam connected to penstocks and outlets ducts. The waterway corresponding to river is diverted in dedicated ways corresponding to penstocks intakes. Have a look to the map to get more information

As many major generator and plants in Europe, Genissiat EIC code is given with ref:EU:ENTSOE_EIC=*

Here is a wide electricity production site built in French Alps. Water is stored in a huge lake and goes through 60 km long tunnels ending into the La Bathie underground power plant.

The choice is made to bring all the stuff inside a unique relation instead of the single power plant cavern. The six francis tubrines are currently under maintenance to step up their output to 100 MW of electricity each. Power is sent to the power grid by the Albertville power substation.

Nuclear

Gravelines is the most powerful nuclear electricity generation site in France. 6 units can be found there, with 910 MW output each.

Nuclear reactors are splitted in two map features with power=generator on the building hosting the steam turbine. The dome hosting the most radioactive parts and the fission reaction can be described as a steam generator, only as a bonus since it's not the place which actually produce electricity, relatively to generator:plant=*
The surrounding way carefully follow the fence all around the power plant.
As many major generator and plants in Europe, Gravelines EIC code is given with ref:EU:ENTSOE_EIC=*